A spark plug is mounted to, for example, an internal combustion engine (engine) and used to ignite an air-fuel mixture in a combustion chamber. Generally, a spark plug includes an insulator having an axial hole, a center electrode inserted into a front end portion of the axial hole, a terminal electrode inserted into a rear end portion of the axial hole, a metallic shell provided externally of the outer circumference of the insulator, and a ground electrode provided at a front end portion of the metallic shell. The ground electrode is adapted to form a spark discharge gap in cooperation with the center electrode. When high voltage is applied to the center electrode, a discharge is generated across the spark discharge gap between the two electrodes, thereby igniting the air-fuel mixture.
The insulator is inserted into the interior of the metallic shell and held by the metallic shell by means of a rear end opening portion of the metallic shell being crimped radially inward in a state in which a stepped portion formed on an outer circumferential portion of the insulator is seated on a taper portion formed on an inner circumferential portion of the metallic shell. At this time, in order to prevent outward leakage of the air-fuel mixture and the like which enter between the metallic shell and the insulator, an annular sheet packing intervenes between the taper portion of the metallic shell and the stepped portion of the insulator (refer to, for example, Japanese Patent Application Laid-Open (kokai) No. 2005-190762).
In recent years, in order to meet demand for implementation of high output of an internal combustion engine, the air-fuel mixture is highly compressed. Thus, in view of reliably preventing outward leakage of the air-fuel mixture and the like for ensuring good gastightness, there is conceived enhancement of a seal between the sheet packing and each of the taper portion and the stepped portion through increase in crimping force applied to the rear end opening portion of the metallic shell.
However, increasing the crimping force may involve large deformation of the sheet packing, potentially resulting in radially inward or outward protrusion of the sheet packing from between the taper portion and the stepped portion. As a result, a radially inward protrusion of the sheet packing may squeeze the insulator, or a radially outward protrusion of the sheet packing may enter between the insulator and the metallic shell, potentially resulting in breakage, such as cracking, of the insulator.
The present invention has been conceived in view of the above circumstances, and an object of the invention is to provide a spark plug for an internal combustion engine in which, while gastightness is improved through increase in crimping force, deformation of a sheet packing associated with increase in crimping force can be restrained and in turn, breakage of an insulator can be more reliably prevented.